Evaluating Latent Tuberculosis Infection Test Performance Using Latent Class Analysis in a TB and HIV Endemic Setting

Given the lack of a gold standard for latent tuberculosis infection (LTBI) and paucity of performance data from endemic settings, we compared test performance of the tuberculin skin test (TST) and two interferon-gamma-release assays (IGRAs) among health-care workers (HCWs) using latent class analysis. The study was conducted in Cape Town, South Africa, a tuberculosis and human immunodeficiency virus (HIV) endemic setting Methods: 505 HCWs were screened for LTBI using TST, QuantiFERON-gold-in-tube (QFT-GIT) and T-SPOT.TB. A latent class model utilizing prior information on test characteristics was used to estimate test performance.

LTBI prevalence (95% credible interval) was 81% (71-88%). TST (10 mm cut-point) had highest sensitivity (93% (90-96%)) but lowest specificity (57%, (43-71%)). QFT-GIT sensitivity was 80% (74-91%) and specificity 96% (94-98%), and for TSPOT.TB, 74% (67-84%) and 96% (89-99%) respectively. Positive predictive values were high for IGRAs (90%) and TST (99%). All tests displayed low negative predictive values (range 47-66%). A composite rule using both TST and QFT-GIT greatly improved negative predictive value to 90% (range 80-97%).

In an endemic setting a positive TST or IGRA was highly predictive of LTBI, while a combination of TST and IGRA had high rule-out value. These data inform the utility of LTBI-related immunodiagnostic tests in TB and HIV endemic settings.